Optical networks designed for Ethernet traffic are becoming more important as the dominance of data over voice services increases. Work in both standards committees and research communities have targeted the transport of 100-Gbit/s Ethernet (100 GE) over wide area networks. Orthogonal frequency division multiplexing (OFDM) is a good transmission format for realizing 100 Gbit/s signal transmission. In recent years, a number of different alternatives have OFDM as a promising method to eliminate the need for optical dispersion compensation in long-haul transmission links. Fiber-optic OFDM systems can be realized either with direct detection optical (DDO) or with coherent optical (CO) detection. Recently, several high data rate OFDM transmission experiments have been reported. Up to 52.5 Gbit/s OFDM signal has been generated and transmitted over 4160 km. But due to the limited bandwidth of the analog to digital converter (A/D) and digital to analog converter (D/A), no 100 Gbit/s OFDM signal has been generated.
The diagrams of FIGS. 1 and 2 show the architecture to generate over 50 Gbit/s OFDM signal in a publication, Sander Jansen et al., 16×52.5-Gb/s, 50-GHz spaced, POLMUX-CO-OFDM transmission over 4,160 km of SSMF enabled by MIMO processing, ECOC 2007: PD. 1. 3. The diagram of FIG. 1 is directly from the Sander Jansen et al. publication and can be reviewed for further details beyond what are necessary here.
In the Sander Jansen et al. technique, each modulator structure consists of two single-ended MZM modulators 202 or MZ to modulate each polarization independently. Subsequently the two POLMUX signals are combined using a polarization beam splitter 208 and the even and odd WDM channels are combined with a 50-GHz inter-leaver. The electrical OFDM channel allocation is illustrated in FIG. 1. Two different frequency RF signals 205, 206 are mixed with data 1 and data 2. After the intensity modulator 202, the electrum spectrum is shown in FIG. 1, while the optical spectrum is shown in FIG. 2. Due to the optical carrier suppression, the carrier is suppressed. Then optical filter or inter-leaver (207) is aligned such that the image band of the OFDM signal is rejected. As you can see in FIG. 2, only one sideband is employed. Because both sidebands have the same information, one sideband has to be rejected. In this way, only 50 Gbit/s OFDM can be generated due to the limited bandwidth of an A/D converter.
Accordingly, there is need for a method to generate over 100 Gbit/s OFDM signals with the limited bandwidth for A/D and D/A converter tolerance.